It is well known that an HID lamp requires some form of ballast to regulate its operating voltage and wattage because of the negative resistance characteristics of the lamp. The usual ballast has been of a reactive type. The general objective of a regulating ballast is to cause the lamp to operate at its design wattage even when the line voltage varies over a range of, for example, plus or minus 10%, or when aging of the lamp causes an increase in lamp operating voltage which, in turn, usually causes an excessive increase in the power dissipated by the lamp.
A magnetic regulator ballast can provide reasonably good control of the lamp wattage over a reasonable range of line voltages and within the allowable range of lamp voltage variations, but it is not capable of holding the lamp wattage at the true design level and also has some other disadvantages including the lack of simple adjustability of the lamp operating wattage which means that it cannot be used as the basis for a dimming ballast or an energy management lamp control circuit. Accordingly, efforts have been made to combine electronic control circuitry with a magnetic regulator to arrive at a so-called "hybrid" ballast which has much improved control. One example of such a circuit is shown in U.S. Pat. No. 4,501,994, Spreadbury.
These efforts have not been entirely successful. The magnetic regulator remains superior as to such considerations as line power factor, lamp operating current crest factor and lumen maintenance, lamp stability at higher line and lamp voltages, tolerance to line voltage dips and other factors. Notably, some hybrid circuits have allowed the lamp wattage to exceed rated values at high line voltage and to fall below ratings at moderately low line voltages, thereby allowing the light output of the lamp to be too low. Others have exhibited poor stability, short lamp life, high losses and other problems including the production of very high line harmonics.